The invention relates to a shadow mask for a color cathode-ray tube and more particularly to a shadow mask made of an iron-nickel alloy which exhibits improved formability and oxidation characteristics.
A conventional shadow mask-type cathode-ray tube comprises generally an evacuated envelope having therein a screen comprising an array of phosphor elements of three different emission color which are arranged in cyclic order, means for producing three convergent electron beams which are directed toward the target and a color-selection structure including an apertured masking plate which is disposed between the target and the beam-producing means. The masking plate shadows the target and, therefore, is commonly called the shadow mask. The differences in convergence angles permit the transmitted portions of each beam to impinge upon and excite phosphor elements of the desired emission color. At about the center of the shadow mask, the masking plate intercepts all but about 18% of the beam currents; that is, the shadow mask is said to have a transmission of about 18%. Thus, the area of the apertures of the masking plate is about 18% of the area of the mask. The remaining portions of each beam which strike the masking plate are not transmitted and cause a localized heating of the shadow mask to a temperature of about 353 K. As a result, the shadow mask thermally expands causing a "doming" or expansion of the shadow mask toward the screen. When the doming phenomenon occurs, the color purity of the cathode-ray tube is degraded. The material conventionally used for the shadow mask, and which contains nearly 100% iron, such as aluminum-killed (AK) steel has a coefficient of thermal expansion of about 12.times.10.sup.-6 /K at 273 K. to 373 K. This material is easily vulnerable to the doming phenomenon.
Modern color television picture tubes are currently made in large sizes ranging from 25 to 27 inch diagonal dimensions and tubes as large as 35 inch diagonal are being produced in small quantities. Many of these tubes feature nearly flat faceplates which require nearly flat shadow masks of very low thermal expansivity.
Invar, an iron-nickel alloy, has low thermal expansivity, about 1.times.10.sup.-6 /K to 2.times.10.sup.-6 /K at temperatures within the range of 273 K. to 373 K.; however, conventional Invar has a high elasticity and a high tensile strength after annealing, as compared to ordinary iron. Additionally, it has proved to be difficult to produce a strongly adherent low reflection oxide coating, on a conventional Invar shadow mask. A dark oxide is desirable to enhance image contrast.